Increase of the emission efficiency and reduction of the threshold current in various light emitting diodes or semiconductor laser devices are essential for high output and improvement of power consumption or the like and are currently being intensively investigated. However, in a nitride compound semiconductor light emitting device which emits blue or green light, the amount of current injected increases as the emission wavelength increases, which causes problems such as a reduction in the emission efficiency and an increase in the threshold current. One cause of these problems is non-uniformity of carriers in an active layer (i.e., a light emitting layer). That is, the energy gap difference between barrier and well layers which constitute a multiquantum well structure increases as the emission wavelength increases. In addition, when an active layer is formed on a c-surface of a GaN substrate, piezoelectric field effects occur in well or barrier layers such that it is difficult for carriers (electrons or holes) to exit a well layer once they have entered the well layer, thus causing non-uniformity of carriers in the active layer (light emitting layer).
An example in which such a phenomenon is represented by numerical calculation is described in Non-Patent Literature 1, IEEE, Journal of Selected Topics in Quantum Electronics Vol. 15 No. 5 (2011) p. 1390. According to this Non-Patent Literature 1, the difficulty for carriers in a well layer to exit the well layer when the emission wavelength is equal to or greater than 400 nm in the case where an active layer is formed on a c-surface of a GaN substrate, and when the emission wavelength is equal to or greater than 450 nm in the case where an active layer is formed on a non-polar surface of a GaN substrate, is illustrated by a relationship between an emission recombination time and the time required for a carrier to escape from a well layer (see FIG. 12). In FIG. 12, “A” represents the behavior of a hole in the case where an active layer is formed on a c-surface of a GaN substrate, “B” represents the behavior of an electron in the case where an active layer is formed on a c-surface of a GaN substrate, “a” represents the behavior of a hole in the case where an active layer is formed on a non-polar surface of a GaN substrate, and “b” represents the behavior of an electron in the case where an active layer is formed on a non-polar surface of a GaN substrate. Typically, carriers move between well layers in a multiquantum well structure having two or more well layers in a very short time of about 100 femtoseconds or less. However, for the above reasons, the time required for a carrier to escape from a well layer is increased and electrons or holes cannot freely move between well layers. As a result, the electron and hole concentrations in each well layer differ from each other and remaining carriers do not contribute to emission, reducing the emission efficiency. In addition, significant differences in the carrier concentration between well layers lead to variations in the emission wavelength and variations in the gain peak (wavelength), which also causes a reduction in the emission efficiency and an increase in the threshold current.